Hitherto, it has been attempted to blend various additives into lubricants for the purpose of improving the performance of lubricants (grease compositions and the like) to be used in rolling parts such as roller bearings in industry. In the case of using roller bearings and the like under high temperature conditions, there is a possibility that the sealed grease composition is oxidized and degraded and the lubricating film is likely to be broken so that the lubricating life span is shortened. Breakage of the lubricating film is likely to occur particularly under high temperature and high speed conditions, and metal contact between the rolling parts occurs and malfunctions that heat generation and frictional wear increase are caused when the lubricating film is broken. For this reason, an amine-based antioxidant, a phenol-based antioxidant, and the like are used in grease compositions to be used in roller bearings and the like as an antioxidant singly or in combination of plural kinds in order to improve high temperature durability. At the same time, organic sulfonates and sorbitan fatty acid esters are also used in the grease compositions as a rust-preventive agent in order to prevent the generation of rust on the rolling parts.
For example, those using a phenol-based antioxidant such as 2,2′-methylenebis(6-t-butyl-4-methylphenol), an amine-based antioxidant such as octylated diphenylamine, a sulfur-based antioxidant such as phenothiazine, and a phosphorus-based antioxidant such as zinc dithiophosphate as an antioxidant have been proposed as lubricants for bearings (see Patent Document 1). It is further described in Patent Document 1 that a rust-preventive agent such as petroleum sulfonate, dinonylnaphthalene sulfonate, or a sorbitan ester can be concurrently used as another additive.
In addition, in roller bearings, there is a possibility that unique flaking accompanied by microstructural changes occurs on the rolling surface at an early stage as sudden acceleration and deceleration are further added to the above high temperature and high speed conditions and the use conditions thus become severer. This unique flaking is different from flaking from the inside of the rolling surface caused by usual metal fatigue, is a fracture phenomenon occurring from a relatively shallow place of the rolling surface, and is considered as hydrogen embrittlement caused by hydrogen. For example, it is considered that premature flaking due to hydrogen embrittlement occurs as hydrogen is generated by the decomposition of grease and then introduced into the steel of the roller bearing. Hydrogen significantly decreases the fatigue strength of steel, thus cracks are generated and propagated around the inside of the roller surface layer at which the alternating shear stress becomes maximum, and this leads to premature flaking even under conditions to be considered as elastic fluid lubrication in which the contact elements are separated from each other by an oil film.
As a method of suppressing unique flaking phenomenon which is accompanied by such microstructural changes and occurs at an early stage, for example, a method in which bismuth dithiocarbamate is added (see Patent Document 2) and a method in which a molybdate and an organic acid salt are added (see Patent Document 3) have been proposed.
In addition, as a grease composition exhibiting excellent heat resistance, mechanical stability, water resistance, rust preventive property, load bearing property, flame retardancy, and the like, those prepared by blending tribasic calcium phosphate and a grease structure stabilizer such as a diethanolamine with a base oil composed of mineral oil and synthetic oil have been proposed (see Patent Document 4).